For analysis of combustion methods in internal combustion engines, it is known to pick up combustion chamber signals via sensors and to subsequently evaluate them. In measurements performed on internal combustion engines, however, it is nearly unavoidable that the combustion chamber signal is disturbed by interferences so that an interference suppression must be performed on the picked-up signal or on the data generated therefrom.
For analysis and optimization of the combustion methods of internal combustion engines and, as the case may be, also for control device calibration, it is customary, for example, to record the pressure developments in the interior of the cylinders via suited pressure pick-ups, charge amplifiers, and fast data acquisition systems. As a consequence of the not always ideal conditions for installation of the pressure sensors and due to external influences such as structure-borne noise signals or structure-borne noise vibrations as caused, for example, by the closing of valves, the measured pressure curve is afflicted by various disturbing influences which will affect the accuracy of the evaluations. It is known to subject the cylinder pressure signal to filtration for this reason.
Possible pulsating vibrations superimposed on the cylinder pressure as well as high pressure gradients such as those occurring in cases of pre-ignition, will, however, be filtered and thus be reduced in amplitude via such a filtration. Incorrect detection of such phenomena entails the danger that the engine may be overloaded and thus damaged. A reduction of the pressure gradient will also prevent a correct determination of combustion noise.
Since these phenomena occur in the range around the maximal pressure, one possibility to avoid the above mentioned side effects lies in not filtering the signal in a uniform manner across the entire crankshaft angle range.
It is known, for example, that the cylinder pressure signal can first be digitalized in a temporally synchronous manner, then transformed to an angular basis, and then smoothed by weighted averaging wherein, for this sliding averaging, the weight function as well as the window width can be varied via the crankshaft angle.
Since the above is a smoothing method that is applied on a signal which is transformed to a crankshaft angle, however, it will have the significant disadvantage of being ill-suited to indicate an exact filter characteristic line or an exact limiting frequency because the temporal distance between the crankshaft angle positions changes with the rotary speed.
According to a further known method, a crankshaft-dependent filtration of the cylinder pressure development is performed that is adapted to specific disturbance variables, wherein, however, the crankshaft information is in turn derived from the cylinder pressure curve. This has the disadvantage that the crankshaft information at a given point of time is known only approximately and that the current changes of the rotary speed caused by the individual cylinders are left entirely unconsidered.
Since the sample frequency on the time basis is normally considerably higher than on the crankshaft basis, the detected combustion chamber signal will lose information as a consequence of the angle-synchronous smoothing. The determination of the crankshaft position from an analysis of the cylinder pressure development is also massively restricted in its accuracy and is not useful for high-quality evaluation of data.